Concrete 101

THE IMPORTANCE OF UNDERSTANDING CONCRETE

Air improves the durability in: environments of freeze/thaw, deicer salts, sulfate (ground water) and alkali reactivity. It also improves the workability of the concrete. The DOT may have certain project specification requirements which require a specific amount of air content at the point of placement or at the discharge end of the pumping.

Air loss can be minimized when pumping by slowing down the “freefall” of the concrete and minimizing the pumping pressure.

Slump is a measure of concrete consistency or fluidity. Consistency is the ability of freshly mixed concrete to flow. For given proportions of cement and aggregates (without admixtures), the higher the slump, the wetter the mix. Four-inch (4”) slump is very common with normal weight concrete and is a good average slump for pumping. Above average slump – due to the addition of water – considerably reduces the strength, durability, and permeability of concrete and can cause segregation.

When possible, admixtures should be used instead of water to achieve higher slump. Slump loss through the pump can negatively affect pumping. Aggregate will absorb large amounts of water when under the pressure of the pump. This can cause a blockage and be completely unpumpable. Such aggregate should be soaked long enough to achieve Saturated Surface Dry (SSD). SSD aggregate cannot absorb any more water.

ACI 304 suggests that the maximum size of angular (crushed stone) coarse aggregate be limited to one-third of the smallest inside diameter of the delivery system.
Lightweight concrete is made using lightweight aggregates of expanded shale, clay and slag. These are used to produce structural lightweight concrete with a freshly mixed unit weight of 90 to 120 pounds per cubic foot. Normal weight concrete is typically 135 to 150 pounds per cubic foot.

Lightweight aggregate is porous with absorption values of up to 20%. This quality requires the aggregate to be thoroughly soaked to minimize the aggregates’ ability to absorb even more water under the pressure of the pump, causing severe slump loss.

Flowable fill is a self-compacting low-strength material with a flowable consistency that is used as alternative to dry fill or backfill material. Some other terms used for this material are Controlled Low-Strength Material (CLSM), controlled density fill, or lean-mix backfill.

Due to the low cement content of flowable fill, the material has a tendency to mechanically set (settle) quickly. When pumping flowable fill, it is important to keep the material moving.